Synesthesia (also spelled synæsthesia or synaesthesia) - from the Greek syn- meaning union and aesthesis meaning sensation, is a neurological condition in which two or more bodily senses are coupled. For example, in a form of synaesthesia known as grapheme-color synaesthesia, letters or numbers may be perceived as inherently colored. In other forms of synesthesia, musical and other sounds may be perceived as colored or having particular shapes. While cross-sensory metaphors are sometimes described as "synesthetic", true neurological synesthesia is involuntary and occurrs in about four percent of the population (1 in 23 persons) across its range of variants (see Simner et al., in press). It runs strongly in families, possibly inherited as an X-linked dominant trait.

Markus Zedler describes in a brief article synesthesia as "a perceptual condition in which the stimulation in one sensory modality elicits a concurrent sensation in another, a perception which is perceived as real."

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How someone with synaesthesia might perceive (n.b., not 'see') certain letters and numbers.

There are many varieties of synesthesia. While almost any sensory pairing is logically possible, certain combinations are much more common than others. In one of the most common forms of synesthesia, grapheme->color synaesthesia, individual letters of the alphabet, as well as numbers, are "shaded" or "tinged" with a color. While no two synesthetes will report the same colors for all letters and numbers, studies of large numbers of synesthetes find that there are some commonalities across letters (e.g., A is likely to be red) (Day, 2005; Simner et al., 2005).

Synaesthetes often report that they were unaware their experiences were unusual until they realized other people did not have them, while others report feeling as if they had been keeping a secret their entire lives. Writer and synaesthete Patricia Lynne Duffy remembers one early experience: "'One day,' I said to my father, 'I realized that to make an 'R' all I had to do was first write a 'P' and then draw a line down from its loop. And I was so surprised that I could turn a yellow letter into an orange letter just by adding a line.'"[1]

Another grapheme -> color synesthete reports: "I often associate letters and numbers with colors. Every digit and every letter has a color associated with it in my head. Sometimes, when letters are written boldly on a piece of paper, they will briefly appear to be that color if I'm not focusing on it. Some examples: "S" is red, "H" is orange, "C" is yellow, "J" is yellow-green, "G" is green, "E" is blue, "X" is purple, "I" is pale yellow, "2" is tan, "1" is white. If I write SHCJGEX it registers as a rainbow when I read over it, as does ABCPDEF." (From a slashdot discussion)

Other synesthetes report that days of the week, months of the year, and numbers have specific spatial locations, or that numbers will have specific genders and personalities.

James Wannerton, a lexical-gustatory synaesthete reports: "Whenever I hear, read, or articulate (inner speech) words or word sounds, I experience an immediate and involuntary taste sensation on my tongue. These very specific taste associations never change and have remained the same for as long as I can remember". "[2]

Given the wide variety of forms of synesthesia, researchers have adopted a convention of indicating the type of synesthesia by using the following notation x -> y, where x is the "inducer" or trigger, and y is the "concurrent" or additional, extra experience. So, for example, seeing letters and numbers (collectively called graphemes) as colored would be indicated as grapheme -> color synesthesia. Similarly, when synesthetes see colors and movement as a result of hearing musical tones, it would be indicated by tone -> (color, movement) synesthesia.

Proof that someone is a synesthete is fairly easy to come by, and hard to "fake". The simplest test involves test-retest reliability over long periods of time, and synesthetes consistently score higher on such tests than non-synesthetes (either with color names, color chips or even a color picker providing 16.7 million color choices). Synesthetes may score as high as 90% consistent over test-retest intervals of up to one year, while non-synesthetes will score 30-40% consistent over test-retest intervals of only one month, even if warned that they are going to be retested (e.g., Baron-Cohen et al., 1996).

More specialized tests include using modified versions of the Stroop effect. In the standard Stroop paradigm, it is harder to name the ink color of the word "red" when it is printed in blue than if the word "blue" was presented in blue. This demonstrates that reading is "automatic". Similarly, if we present a grapheme-color synesthete with a four that they see as red, but present it in blue ink, they are slower to identify the ink color. Note that this is not because they cannot see the blue ink, but rather that the same sort of "response conflict" that is responsible for the standard Stroop effect is also occuring between the color of the ink and the automatically induced color of the grapheme. Similar variants of the Stroop can be devised where, for example, someone is asked to name a red color patch while listening to a tone that produces a blue sensation (Ward et al., 2006), or to identify a bitter taste while hearing a musical interval that induces a sweet taste (Beeli et al., 2005).

An example of a test used to demonstrate the reality of synesthetic experiences (from Ramachandran and Hubbard, 2001).

Finally, studies of grapheme-color synesthesia have demonstrated that synesthetic colors can improve performance on certain visual tasks, at least for some synesthetes. Inspired by tests for color blindness, Ramachandran and Hubbard (2001) presented synesthetes and non-synesthetes with displays composed of a number of 5s, with some 2s embedded among the 5s. These 2s could make up one of four different shapes; square, diamond, triangle or diamond. For a synesthete who sees 2s as red and 5s as green, their synesthetic colors would be expected to help them to find the "embedded figure". This is indeed what was found. Subsequent studies have explored these effects more carefully, and have found that 1) there is substantial variability among synesthetes and 2) while synesthesia is evoked early in perceptual processing, it does not occur prior to attention.

Over the last 100 years there have been a number of case studies reported in the scientific literature and there have been references in the biographies of notable artists indicating that they experienced the condition.

Estimates of the prevelance of synesthesia have varied widely (from 1 in 20 to 1 in 20000). However, these previous studies all suffered from the methodological shortcoming of relying on self-selected samples. That is, the only people included in those earlier studies were those who reported their experiences to the experimenter. Simner et al., (in press) conducted the first random population study, arriving at a prevalence of 1 on 23. Recent data (Day, 2005; Simner et al., in press) suggests that grapheme -> color, and days of the week -> color variants are most common.

In general very little is definitively known about the overall cognitive traits associated with synesthesia (or, indeed if there are any cognitive traits that are consistently associated with synesthesia). Some studies have suggested that synsthetes are unusually sensitive to external stimuli (see, e.g., Cytowic, 2002). Other possible associated cognitive traits include left-right confusion, difficutlies with math, and difficulties with writing (Cytowic, 2002). However, synesthetes may be more likely to participate in creative activities (Rich et al., 2005), and some studies have suggested a correlation between synesthesia and creativity (Domino, 1989; Dailey et al., 1991). Other research has sugggested that synesthesia may contribute to superior memory abilities (Luria, 1968; Smilek et al., 2002). However, it is unclear whether this is a general feature of synesthesia or whether it is true of only a small minority. This is a major topic of current and future research.

This picture is used as a test to demonstrate that people may not attach sounds to shapes arbitrarily: A remote tribe calls one of these shapes Booba and the other Kiki. Decide which is which and then click the image to check your answer.

Researchers study synaesthesia not only because it is inherently interesting, but also because they hope that studying synaesthesia will offer new insights into other questions, such as how the brain combines information from different sensory modalities, referred to as cross-modal perception and multisensory integration. One example of this is the "kiki/bouba effect" (see image at right). This example was first explored by the Gestalt psychologistWolfgang Kohler. With individuals on the island of Tenerife, Kohler showed a similar preference between shapes called "takete" and "maluma". Recent work by Daphne Maurer and colleagues has shown that even children as young as 2.5 (too young to read) show this effect. Ramachandran and Hubbard (2001) have argued that this effect might be the neurological basis for sound symbolism, in which sounds are non-arbitrarily mapped to objects and events in the world.

Similarly, synaesthesia researchers hope that, because of their unusual conscious experiences, the study of synaesthesia will provide a window into better understanding consciousness and in particular on the neural correlates of consciousness, or what the brain mechanisms that allow us to be conscious might be. In particular, some researchers have argued that synaesthesia is relevant to the philosophical problem of qualia (see, e.g., Gray et al., 2002; Gray et al., 1997; Ramachandran & Hubbard, 2001), since synaesthetes experience additional qualia evoked through non-typical routes.